The present invention relates to a driving method for a liquid crystal display device, and more particularly to a driving method for a liquid crystal display device in which a liquid crystal exhibiting a cholesteric phase is sandwiched between two substrates having electrodes arranged in a matrix form on their surfaces, and which creates a display by changing the state of the liquid crystal by a voltage applied across the electrodes.
In a liquid crystal display device with a cholesteric liquid crystal or chiral nematic liquid crystal sandwiched between two substrates, a display is created by switching the state of the liquid crystal between a planar state and a focal conic state. When the liquid crystal is in the planar state, light with wavelength xcex=Pxc2x7n is selectively reflected, where P is the helical pitch of the cholesteric liquid crystal and n is the average refractive index of the liquid crystal. In the focal conic state, when the selective reflection wavelength of the cholesteric liquid crystal is in the infrared region, the liquid crystal scatters light, and when the reflection wavelength is shorter than that, the liquid crystal transmits visible light. Accordingly, by setting the selective reflection wavelength within the visible light region, and providing a light absorbing layer on the side of the display device opposite the side thereof viewed by the observer, selective reflection color can be displayed in the planar state, and black in the focal conic state. On the other hand, by setting the selective reflection wavelength within the infrared region, and providing a light absorbing layer on the side of the display device opposite the side thereof viewed by the observer, black can be displayed in the planar state, since the light of wavelengths in the infrared region is reflected but the light of wavelengths within the visible spectrum is transmitted through the liquid crystal, and white can be displayed in the focal conic state because of light scattering.
Here, let Vth1 denote a first threshold voltage for unwinding the twist of the liquid crystal exhibiting the cholesteric phase; then, after the voltage Vth1 is applied for a sufficient amount of time, when the voltage is lowered below a second threshold voltage Vth2 smaller than the first threshold voltage Vth1, the planar state results. When a voltage larger than Vth2 but smaller than Vth1 is applied for a sufficient amount of time, the focal conic state results. These two states are stable even after the applied voltage is removed. Further, a phase in which these two states are mixed is known to exist, and it is known that a gray scale display is possible (refer to U.S. Pat. No. 5,384,067).
In this way, since the liquid crystal exhibiting the cholesteric phase has the memory characteristic that can retain the display state in the absence of an applied voltage, a desired image or character can be displayed by driving the display device, divided into many pixels, by passive matrix addressing. However, since this kind of liquid crystal has a hysteresis characteristic, for the same applied voltage the display state can differ depending on the previous state of the liquid crystal.
To overcome this deficiency, the present assignee proposed a driving method in which, after resetting the state of the liquid crystal to the homeotropic state by applying a voltage greater than Vth1, a plurality of write pulse voltages are applied and the state of the liquid crystal is selected by the magnitude of the applied voltage. This driving method applies pulse waveforms such as shown in FIG. 37, to the liquid crystal. Of the three pulses shown, the first pulse 401 is the pulse for resetting the state of the liquid crystal to the homeotropic state, and has a pulse width P1 and voltage V1. The second and third pulses 402 and 403 are the pulses for selecting the state of the liquid crystal, and both have the same pulse width P3 and voltage V2. The pulses 401, 402, and 403 are separated from one another by a wait time P2 during which no voltage is applied. The wait time P2 between the first and second pulses 401 and 402 is the time necessary for the liquid crystal to change from the homeotropic state to the planar state. The wait time P2 between the second and third pulses 402 and 403 is necessary to separate the pulses 402 and 403. In this driving method, the reflectivity of the device is a function of voltage, and gray scale can be reproduced by controlling the second and third pulse voltage V2.
An exemplary object of the present invention is to provide a novel and useful driving method for a liquid crystal display device, that suppresses the degradation of display quality caused by the effect of the hysteresis, as in the above-described driving method, and achieves further improvements in the characteristics of the device. Particularly, it is an object of the invention to provide a driving method for a liquid crystal display device, that achieves a further reduction in driving time.
To achieve the above objects, in the driving method according to the present invention, liquid crystals in all the pixels are first reset at once to the focal conic state that requires a long time for selection, and then a select voltage is applied in sequence to the liquid crystal forming each pixel, thereby selecting the display state of the liquid crystal in every pixel.
According to the present invention, since all the pixels are simultaneously reset to the focal conic state, the long selection time required to select the focal conic state occurs only once in one frame. This improves the update speed in passive matrix driving.